Over 100 Km Ultra-long Distance Vibration Detection And Extraction Based On Phase-sensitive Optical Time Domain Reflectometer

Distributed optical fiber sensing(DOFS)is a unique sensing technology which has wide application scenarios.DOFS uses fiber as transmission media and sensitive detectors simultaneously.It has the advantage of simple structure,high sensitivity,resistance to corrosion and it’s free from electromagnetic interference.Phase sensitive optical time domain reflectometer(Φ-OTDR)works by detecting the phase change in the backscattering light,which fits for vibration signal detection.However,if the sensing distance is more than 100 km,several issues occurred in Φ-OTDR vibration signal detection.The optical signal amplitude is under restriction,and its attenuation increases rapidly as the sensing distance grows.The amplitude is too low to be detected under ultra-long sensing distance.Besides,due to the influence of system device noise,the environment noise and random attenuation effect,the SNR is very low under ultralong sensing distance,which increases the difficulty of vibration detection.The regular schemes for increasing the sensing distance is putting optical amplifier in the middle of fiber segments or putting amplifier in the front and end of sensing fiber,realizing a dual-end detecting structure.However,those schemes increase the complexity of the detecting system.Besides,in practical application scenario,optical amplifier may not be able to be put in the middle of fiber segments.Therefore,single-end ultra long distance Φ-OTDR vibration signal detection scheme has high research and application value.In this paper,over 100 km ultra-long distance single-end vibration detection system based on Φ-OTDR is studied.By combining optical amplification method and electrical signal processing method,an optical-electrical sensing scheme is raised to solve the low amplitude and low SNR problem in single-end ultra-long distance Φ-OTDR vibration detection system.Aiming at the problem that the signal amplitude is too low to satisfy the system dynamic range requirement,this paper combines hybrid optical amplification and sectional electrical amplification to raise the amplitude level.For optical signal,the hybrid optical amplification is implemented by combining EDFA and Raman amplifier to raise the basic amplitude of the optical signal in detection amplitude restriction.The sectional electrical amplification is used to amplify the low amplitude signal section,raise the long distance signal amplitude level further.Aiming at the problem that the SNR is ultra low in long distance detection,on the basis of the hybrid optical amplification which raised the basic signal SNR,Hilbert amplitude demodulation and adapted matched filter are combined as electrical denoising technique which compensates the random attenuation and raises the SNR rapidly.This paper analyzes the problems in ultra-long distance vibration detection firstly,then raises a single-end ultra-long distance vibration detection scheme and introduces the theories of related techniques involved.Furthermore,simulation analysis is taken to analyze the optical-electrical sensing scheme.Finally,the single-end ultra-long distance Φ-OTDR vibration sensing system is implemented and experimented,verifying the conclusion made by simulation analysis and analyzing the system performance.The optical-electrical signal processing scheme used to Φ-OTDR vibration sensing system raised by the paper has detected ultra-long distance vibration successfully in the experimental environment.The sensing distance reaches 147 km,with frequency response 95 Hz and spatial resolution 40 m.The system has simple structure and good detection performance.